In recent years, use of digital color images has increased abruptly along with the popularization of digital cameras. Photo print techniques for satisfactorily printing these images have been extensively developed. On the other hand, in the field of silver halide photos, it has been prevalent to take monochrome photos using vintage cameras. Monochrome photos, unlike color photos, express an object's texture by subtle flavor and expressive power, and are used as an expressive means different from that of color photos. Digital monochrome images are not so currently prevalent compared to color photos. If digital cameras are used to produce expressive images of the same sort as monochrome silver halide photos in the future, however, expansion of the usage of digital monochrome images is expected.
In general, a monochrome image is printed by forming an image using a black color agent (ink or toner). When an image is formed using a black color agent alone, however, the color characteristics of the black color agent practically determine the tincture of a printed image. Hence, the tincture of a printed image cannot be controlled in order to be reproduced desirably.
A monochrome image is also often formed by a so-called “composite black”, using color agents such as, inter alia, cyan (hereinafter abbreviated as C), magenta (hereinafter abbreviated as M), yellow (hereinafter abbreviated as Y), in addition to black (hereinafter abbreviated as K). In these cases, by combining color agents at an appropriate ratio, the tincture of a monochrome image can be desirably reproduced. Also, by changing the ratio of combined color agents, the tincture can be adjusted.
Furthermore, when a color printer is used to print a monochrome image, the tincture cannot be adjusted unless the printer has a special adjustment function. Hence, when a monochrome image is outputted with a desired tincture, image data is converted into R, G, and B color component signals, which are to be adjusted.
In the above prior art, however, when the tincture is adjusted by adjusting the color agent amounts or color component signal values, because the relationship between the adjustment amounts and print colors is not always constant, an unexpected adjustment result is often obtained. Some adjustments may also lose a tincture balance at a specific gray level, and the tincture may appear in some disproportionate fashion. For example, when a tinge of yellow is to be enhanced by increasing the amount of a Y color agent or decreasing a B signal value, the tincture of middle lightness has nearly no change, but an image with excessively yellowish highlighting may be formed. Furthermore, some adjustments may often change the brightness of an image.